The invention presented herein relates to circuitry having a plurality of manually actuated switches for providing a control signal output in response to actuation of a preselected combination of the switches in a preselected sequence.
Various circuits have been devised having a plurality of manually actuated switches for providing a control signal output in response to the actuation of a preselected combination of the switches in a preselected sequence. Such circuits include the use of flip-flop circuit portions which are connected in series in such a manner that operation of the second and subsequent flip-flop circuit portions in the series is dependent on the operation of a preceding flip-flop circuit portion. Such operation is attained only by actuation of the preselected combination of switches in accordance with a preselected sequence. In such prior arrangements, wherein operation of a selected switch for the combination provides an input signal to a flip-flop, a selected switch can not be used more than once in the preselected combination. With such a limitation, the number of possible combinations of a plurality of switches is limited. It is desirable that the number of combinations available for selection from a plurality of switches be as large as possible to reduce the chances for an unauthorized user to arrive at the preselected combination of switches needed for operation of the circuitry. Further, not being able to use a switch twice in a preselected combination, precludes full use of letters of the alphabet assigned to the plurality of manually actuated switches since it is not possible to designate a preselected combination by a word or series of letters requiring the use of a letter more than once, such as in Bill, book, and loop, for example.
While various circuitry is known which employs coded manual switching for producing a control signal, the control signal can be repeated only by a second entry of the required coded switch actuations. This deficiency limits the use of such circuitry.